Carbon fibres (CF)—discovered for the first time by Edison in 1879 upon the carbonisation of a cotton thread, while searching for a filament suitable for incandescent lamps—appeared on the market only in 1960 through a manufacturing process devised by William Watt for the Royal Aircraft in the UK starting from the transformation of a polyacrylonitrile fibre (PAN).
Carbon fibres consist of thin filaments, continuous or of predetermined length (staple fiber), having a diameter of 5-10 μm, consisting mainly of carbon atoms. Carbon atoms are mutually bonded in a crystal matrix, wherein the individual crystals are aligned, to a smaller or larger extent, along the longitudinal axis of the fibre, thus imparting to the fibre an extraordinarily high resistance compared to the size thereof.
Various thousands of carbon fibres are then mutually gathered to form a thread or a tow (or roving) which can then be used as it is or woven in a loom to form a fabric. The yarn or fabric thus obtained are impregnated with resins, typically epoxy resins, and then moulded to obtain composite products characterised by high lightness and resistance.
Carbon fibres represent the transition point between organic and inorganic fibres; as a matter of fact, they are produced starting from organic fibres which are modified by thermal treatments and pyrolysis, during which first a reorientation of the molecular segments within the individual fibres is caused and subsequently, at higher temperatures, the removal of oxygen, hydrogen and of most of the nitrogen occurs, so that the final fibre consists to over 90% and up to 99% of carbon and for the rest of nitrogen.
Together with the availability of glass fibres, the availability on the market of carbon fibres has given rise to the use of composite materials to an ever growing extent. With the use of carbon fibres, in particular, it has been possible to devise composite materials having advanced mechanical performances for uses initially for the military and/or aeronautic sectors, considering the high cost of this material, and later—with the improvement of the manufacturing techniques and resulting cost reduction—also for the products of the energy industry (pressurised tanks, wind generator blades, fuel batteries, off-shore platforms), of the transport industry (trains, cars, boats) and of the leisure industry (tools and equipment for practising sports). While for this last application sector already today the market appears fully developed, in the aeronautical sector, and especially in the industrial sector, in the next 5-year period a sharp demand increase is expected and hence the need to extend the existing pool of manufacturing plants.
Carbon fibres are currently manufactured by modification of artificial fibres (rayon industrially, lignin experimentally) or synthetic fibres (polyacrylonitrile for at least 90% of the world output, but also PBO and experimentally other thermoplastic fibres) or of residues of the distillation of oil or tar (pitch). The first ones are traditionally called PAN-derived carbon fibres, while the second ones are called pitch-derived carbon fibres. This last type of fibres is often improperly referred to as “graphite fibres”, even though of course they are not fibres obtained from graphite, to stress the fact that when such fibres undergo a thermal treatment above 2000° C., they finally exhibit a carbon atom arrangement very similar to that typical of graphite and a substantial absence of other elements in the reticule.
In the case of PAN-derived carbon fibres, a sector in which the present invention is set, the starting polyacrylonitrile fibre (the so-called precursor) must be characterised by a suitable chemical composition, by a special molecular orientation and by a specific morphology, so that a final carbon fibre with satisfactory features may be obtained from the same. The chemical composition is important also for the purpose of controlling the exothermic level of the cyclisation reaction of the —CN, equal to 18 kcal/mole, a reaction which represents the first processing step of the polyacrylonitrile fibre. In the textile-derived plants, the precursor is typically mass-produced and the individual fibres are collected in bundles or tows containing up to 300,000 individual filaments; the smaller tows produced in this type of plants contain for example 48,000 filaments (so-called 48K). At the same time, plants exist which were devised specifically for manufacturing low-denier tows, where production occurs on a small or medium scale with the manufacture of tows of 1K, 3K, 6K and 12K. In this case the individual tows can be mutually gathered to form larger ones, for example 24K or 48K tows, at the end of the carbonisation process. The carbon fibres produced in the first type of plants have a lower manufacturing cost, given by the high productive capacity of the same, but they have a smaller degree of regularity, and they are hence better suited for industrial uses. The carbon fibres produced in the second type of plants are instead more regular and more appreciated by the aeronautical industry, where there is already a consolidated habit of using smaller carbon fibre tows.
The cyclisation reaction of the PAN fibres represents, as stated above, the first step of the carbonisation process. It is conducted in air, at 200-295° C. (220-275° C. in current practice) for a few hours, and leads to a black, fireproof material, the so-called oxidised PAN, which exhibits rather poor mechanical properties and is meant—as it is—for the production of protective clothing, fireproof padding or, in carbon-carbon composites, of heavy-duty brakes (for aircrafts, racing cars and high-speed trains).
During the cyclisation step at 200-295° C. it is very important to check for fibre retraction, since in this step the alignment of molecular segments along the fibre axis is determined, on which orientation the final elastic modulus of the carbon fibre depends. The molecular orientation imparted to the original acrylic fibre affects the toughness and the elastic modulus of the final carbon fibre; however, the orientation degree must not be excessively high because in this case defects are introduced, both superficially and within the fibre.
The PAN fibre thus oxidised hence undergoes a subsequent carbonisation process, generally conducted in an inert atmosphere, during which the removal of foreign atoms from the carbon structure occurs with the development of the final graphite structure. The carbonisation process generally occurs in two steps: a first low-temperature step (350-950° C., 400-900° C. in current practice) and a second, high-temperature step (1000-1800° C., 1000-1450° C. in current practice). During all the steps of the carbonisation process hence HCN, NH3 and N2 develop and CO, CO2 and H2O may also develop depending on the amount of O2 that the PAN fibre has bound during the cyclisation at 200-295° C. in air. After the thermal treatment at over 1000° C. the PAN fibre has turned into a carbon fibre containing about 95% of carbon and 5% of nitrogen. During the carbonisation process the fibre is subject to a transversal shrinking which implies a diameter reduction with loss of about 50% of the initial weight thereof; the corresponding longitudinal shrinking is instead nearly fully mechanically hindered, with the corresponding development of a greater molecular orientation which contributes to the improvement of mechanical properties.
Downstream of this process a further pyrolysis treatment may be provided at temperatures ranging between 2000 and 2600° C., of course always in the absence of reactive gases, which takes the name of graphitisation process, during which the residual nitrogen percentage is expelled and the carbon contents of the fibres rise to over 99%. The carbon fibres which have undergone this further treatment exhibit even better mechanical properties, however at much higher costs, and are hence reserved to special uses.
At the end of the carbonisation process, the carbon fibres undergo a cleaning surface treatment and a treatment for attaching functional groups, for the purpose of easing the adhesion of the fibres to the resin matrix in the subsequent forming of composite materials; for this purpose many manufacturers use an electrolytic oxidation process. Finally, on the fibre thus treated, a sizing or finish is applied, for the purpose of minimising the damage deriving from the winding into the bobbin and to further improve fibre adhesion to the resin matrix into which it is meant to be embedded.